The correct transformation of the sentence into Indirect Speech is: (1) I advised my brother not to tell a lie.

The original sentence is a direct speech in which the speaker is directly conveying advice to their brother. In the reported speech, it's crucial to accurately reflect the meaning, intent, and context of the original statement. In this case, the speaker is advising their brother not to tell a lie, which implies a recommendation or counsel.

Therefore, in indirect or reported speech, the reporting verb should be changed to "advised" to maintain the meaning. Here's the breakdown of the sentence transformation:

Original direct speech: "I said to my brother, 'Don't tell a lie.'"

Reported speech: "I advised my brother not to tell a lie."

In the reported speech, "advised" conveys the idea that the speaker provided guidance or counsel to their brother. This accurately represents the original sentence's intention and meaning, making it the most appropriate choice for transforming the sentence into indirect speech.